The establishment of segments during Drosophila embryogenesis provides an excellent system for studying how spatially coherent patterns arise during development and how genes function and interact to specify cell fates. Odd-skipped (odd)is one of eight segmentation genes in the pair-rule class, which occupies the central position among the three hierarchical classes which regulate embryonic segmentation. Genetic studies indicate a unique and unusual interaction for the odd gene during development. Understanding this role is essential for a global view of segmentation. The broad goal of this research is to understand the genetic regulation of segmentation and how the odd gene interacts with other genes to achieve this regulation. The specific aims are four: 1) Characterize the structure of the odd-skipped locus. This involves completing the sequence of the gene, determining the upper limit of genomic sequences required for odd function, isolating additional mutant alleles, and determining how mutations affect the structure and function of the gene. 2) Define the spatial patterns of odd expression. Antibodies to the gene product will be used to identify odd-expressing cells in the developing embryo; the relationship of this pattern to that of other segmentation genes will be determined using double-label techniques. 3) Analyze regulatory interactions between odd-skipped and other segmentation genes. The effects of mutations in other segmentation genes on the pattern of odd expression will be determined. The role of odd as a repressor of the engrailed gene will be examined using various odd alleles, selected double mutants, and embryos in which spatially indiscriminate odd expression has been induced. 4) Characterize genes which are structurally related to odd-skipped. The structure, chromosomal locations, and expression patterns of several genes which share a highly conserved "zinc finger" motif will be determined; functional similarities to odd will be assessed by isolation of mutants, and by testing chimeric genes in which homologous regions have been interchanged. These studies will contribute to our understanding of an important problem in developmental biology, namely how a group of regulatory genes interact to determine spatial patterns and specify cell fates in the developing embryo. They have significance beyond Drosophila development, since similar regulatory networks probably operate to control development of mammalian embryos.